The present invention generally pertains to broadcast communication systems in which a frequency or phase modulated communication signal is upconverted to a carrier frequency for transmission and downconverted upon reception for demodulation, and is particularly directed to eliminating phase noise and drift that occurs incident to such up and down conversion.
In a typical broadcast communication system, a baseband information signal is frequency or phase modulated at a given modulation frequency and the modulated information signal is mixed with an RF signal at a carrier frequency from a local oscillator in a transmitter to provide an upconverted signal for transmission; and upon reception by a receiver the transmitted signal is mixed with an RF signal from a local oscillator in the receiver to provide a downconverted signal at an intermediate frequency for demodulation by the receiver to provide the information signal at the baseband frequency. Frequency errors inherently present in imperfect local oscillators cause phase noise and drift to be present in the signals provided by the local oscillators. In the prior art such phase noise and drift are reduced by constructing the local oscillators to more rigid specifications to reduce the frequency errors therein. However, such a solution to the phase noise and drift problem significantly increases the cost of the transmitter and receiver without completely eliminating the problem.